A multiple-input multiple-output (MIMO) system uses multiple transmit antennas and multiple receive antennas to improve efficiency of data transmission and reception. A MIMO technology includes spatial diversity and spatial multiplexing. The spatial diversity is a technique in which the multiple transmit antennas transmit the same data and thus transmission reliability increases. The spatial multiplexing is a technique in which the multiple transmit antennas simultaneously transmit different data and thus data at high data rate can be transmitted without increasing a system bandwidth.
In conventional wireless communication system, one base station (BS) provides services to a plurality of user equipments (UEs). In order to efficiently provide a data service to the plurality of UEs, the BS schedules and transmits user data according to a channel condition. For this, the UEs transmit a channel quality indicator (CQI) to the BS in a subframe. The subframe is a scheduling unit of transmission. User data and control information are carried together in the subframe. In the subframe, a channel for carrying the user data is referred to as a data channel and a channel for carrying the control information is referred to as a control channel.
The subframe has a fixed interval which is called as transmission time interval (TTI). Amount of radio resources for the data channel and the control channel are restricted by each other in the subframe. When large amount of radio resources are allocated to the control channel, that of radio resources for the data channel is decreased, and thus a data rate is decreased. Therefore, amount of radio resources for the control channel is smaller than that of radio resources for the data channel in order to improve the data rate.
The multiple antenna system can transmit multiple data streams on a plurality of spatial layers. To efficiently scheduling the plurality of spatial layers, a BS needs to know various control information. A CQI indicates downlink channel condition. If a CQI for each of the plurality of spatial layers is reported, the number of CQI which is transmitted on the control channel by a UE increases as much as that of the spatial layers. As a result, amount of radio resources for the control channel increases. In addition, a precoding matrix indicator (PMI) and a rank indicator (RI) are also required to be reported to the BS in the multiple antenna system. The PMI is used to determine a precoding matrix for the BS to perform precoding. The RI is used to determine the number of spatial layer.